1. Technical Field
The invention relates to a capacitive touch panel for detecting an input operation in a noncontact manner made to a sensing electrode arranged on an insulating panel based on increase of the stray capacitance of the sensing electrode.
2. Related Art
A capacitive touch panel is known as a pointing device for designating an item such as an icon displayed on a display of an electronic device. An electrostatic capacitance changes near a position on an entry screen as an input unit such as a finger is approaching. By making use of this phenomenon, the capacitive touch panel can detect an input operation in a noncontact manner based on the change of the electrostatic capacitance even if the capacitive touch panel is placed on the rear side of the display.
A conventional capacitive touch panel includes a large number of insulated X electrodes and a large number of insulated Y electrodes arranged in a matrix on an entry screen such that the X electrodes and the Y electrodes cross each other. An electrostatic capacitance changes between each of the X electrodes and a Y electrode crossing each other near a position where an input unit such as a finger approached, thereby detecting an input operation made at the positions of the X and Y electrodes having changed in electrostatic capacitance (as disclosed in paragraphs 0017 to 0031 of the specification and FIG. 1 of Japanese Patent Application Publication No. 2005-337773).
In the capacitive touch panel disclosed in Japanese Patent Application Publication No. 2005-337773, a predetermined pulse voltage is applied sequentially to a large number of Y electrodes to scan the Y electrodes. The voltage of an X electrode that crosses the Y electrode having received the pulse voltage is detected. In response to approach of an input unit such as a finger to an insulating panel, an electrostatic capacitance changes between X and Y electrodes crossing each other at a position where the input unit is approaching. Accordingly, based on the position of the X electrode having changed in voltage as a result of the change in the electrostatic capacitance and the position of the Y electrode having received the applied pulse voltage at that time, the position of the operation made by the input unit on the insulating panel is detected.
If an entry screen of an insulating panel is increased in area, the number of X electrodes and the number of Y electrodes subjected to detection of the change of an electrostatic capacitance are increased in response to the increase of the input area. This results in a longer scanning cycle during which the crossing points of respective electrodes are scanned, making it impossible to detect an input position in a short time. In addition to the necessity to provide means for applying a pulse voltage, scanning of the large numbers of X and Y electrodes arranged in a matrix requires the use of multiplexers in number corresponding to the numbers of the X and Y electrodes. There is the problem in that the circuit configuration is complicated and increased in size.
Accordingly, a method of detecting an unknown electrostatic capacitance at an input position based on a time constant between the electrostatic capacitance and a known resistance value is known as means for detecting the change of the electrostatic capacitance (stray capacitance) of a sensing electrode with a simpler circuit configuration. In this detection method, a sensing resistor R is connected in series or in parallel with a capacitor C of an electrostatic capacitance that is an unknown capacitance to form a CR time constant circuit. A predetermined voltage Vdd is applied to one end of the sensing resistor R, or one end of the sensing resistor R is grounded. The electric potential of the capacitor C that increases or decreases depending on a time constant rc determined by the electrostatic capacitance c of the capacitor C and the resistance value r of the sensing electrode R is compared with a predetermined threshold potential. Then, the magnitude of an electrostatic capacitance is determined based on charging or discharging time until the threshold potential is achieved. The stray capacitance of a sensing electrode disposed on an insulating panel (electrostatic capacitance between the sensing electrode and the ground) increases to make charging and discharging times longer in response to approach of an input unit such as a finger. Accordingly, by employing the aforementioned detection method, charging and discharging times until the capacitance of the sensing electrode becomes the same as a predetermined threshold potential are measured, and the measured times are compared with charging and discharging times obtained when no input operation is made. As a result, the presence or absence of an input operation made near the sensing electrode can be determined.
However, a stray capacitance c increases only by some picofarads from about 10 pF in response to approach of a finger to a sensing electrode. Accordingly, a time constant changes only by 10 μsec even if the detection method using the CR time constant circuit includes sensing resistors of 10 MΩ connected in series in order to detect an electrostatic capacitance to increase by 1 pF, for example. This makes it quite difficult to detect an input operation made to a sensing electrode directly from comparison of charging times or discharging times until a threshold potential is achieved. This problem may be solved by increasing the resistance value of the sensing resistor further. However, this in turn generates the condition of a high impedance close to an insulated condition to cause flow of a sensing current in a microcomputer and the like for applying a sensing voltage, making detection impossible. In view of these facts, the following electrostatic capacitance detection method of a charge transfer scheme is suggested (as disclosed in paragraphs 0014 to 0020 of the specification and FIG. 2 of Japanese Patent Application Publication No. 2009-70004). In this electrostatic capacitance detection method, a capacitor of a higher capacitance is prepared, and electric charges stored in a stray capacitance are repeatedly transferred to this capacitor to compare charging times of the capacitor.
The electrostatic capacitance detection method of a charge transfer scheme will be described next by referring to FIGS. 5 and 6. A capacitor C1 shown in FIG. 5 of a small capacitance c1 is used for detection of capacitor change. As an example, the capacitor C1 has a minute stray capacitance generated between a finger of an operator and a pattern. One side of the capacitor C1 is grounded through the operator. The capacitor C1 is charged with a charge voltage Vdd while SW1 on the opposite side is ON. A capacitor C2 of a capacitance c2 sufficiently higher than the electrostatic capacitance of the capacitor C1 is connected in parallel with the capacitor C1 through SW2.
In a detection circuit of the aforementioned structure, SW1 and SW2 are turned ON and OFF, respectively, to charge the capacitor C1 with the charge voltage Vdd in a first step. After the charging, SW1 and SW2 are both turned OFF in a second step. The voltage V1 of the capacitor C1 is equal to Vdd in the second step. Next, in a third step, SW1 and SW2 are turned OFF and ON, respectively, so that electric charges stored in the capacitor C1 are transferred in part to the capacitor C2. Then, SW1 and SW2 are both turned OFF again in a fourth step. In the fourth step, the voltages V1 and V2 of the capacitors C1 and C2 are equal to each other.
The voltage V2 of the capacitor C2 after repeating the processes of the first to fourth steps the number of times N is expressed as V2=Vdd×(1−c2/(c1+C2)N). Further, the charge voltage Vdd and the capacitance c2 of the capacitor C2 are known here. Accordingly, the electrostatic capacitance c1 of the capacitor C1 to be detected can be obtained by determining the number of times N until the voltage V2 of the capacitor C2 reaches a threshold potential Vref shown in FIG. 6 that is set to be half the charge voltage Vdd.
As shown in FIG. 6, the repeat count N becomes small until Vref is achieved, with the increased electrostatic capacitance c1. Accordingly, in a capacitive touch panel that has only to detect approach of an input unit to a sensing electrode, a threshold Nref of a repeat count is set, for example, to 1100 in the figure. It is determined that a stray capacitance of 10 pF or higher is generated in response to approach of a finger making an input operation if Vref is achieved by a repeat count lower than the threshold Nref, thereby detecting the input operation to a sensing electrode.
The conventional capacitive touch described in Japanese Patent Application Publication No. 2005-337773 involves the application of a pulse voltage to each of all the sensing electrodes, and accordingly, it desirably employs a detection method with a CR time constant circuit capable of detecting changes of an electrostatic capacitance in a shorter time by simultaneously charging or discharging the stray capacitances of all the sensing electrodes. Meanwhile, a stray capacitance changes slightly in response to an input operation, making it difficult to detect this change based on a difference in charging or discharging times until a threshold potential is achieved. This change can be detected only by being made greater by the charge transfer scheme described in Japanese Patent Application Publication No. 2009-70004.
However, this charge transfer scheme involves controls of the operations of SW1 and SW2 1000 times or more in order to detect one change of an electrostatic capacitance, making it impossible to detect an input operation made near a sensing electrode in a short time based on the change of a stray capacitance.
The invention has been made in view of the aforementioned conventional problems. It is an object of the invention to provide a capacitive touch panel capable of detecting changes of the stray capacitances of a large number of sensing electrodes simultaneously and also capable of detecting an input operation to a sensing electrode based on charging and discharging times until a threshold potential is achieved even if the stray capacitances change slightly.
It is also an object of the invention to provide a capacitive touch panel capable of selecting a method of controlling a voltage with a higher degree of detection accuracy according to a charge voltage Vdd or a threshold potential to be compared with the electric potential Vc of a sensing electrode.